With considerable increase in consumption of fossil fuels, demand for alternative energy or clean energy is rapidly increasing. As one form of such demand, electric power generation and storage technologies using electrochemical reaction are actively being studied.
A representative example of electrochemical elements using electrochemical energy is a secondary battery and application areas of the secondary battery are continuously increasing.
Demand for secondary batteries as an energy source is rapidly increasing and, especially, a lithium secondary battery having high energy density and voltage, long lifespan, reduced self-discharge rate, etc. among the secondary batteries is now commercialized and widely used in the related art. Recently, owing to considerable increase in demand for mobile electric/electronic devices, use of the secondary battery is also considerably expanded. In particular, lithium secondary batteries are also an important part of the foregoing environments.
Moreover, since mobile electric/electronic devices are continuously evolved into smaller and more functional forms, batteries used for such devices also require high performance, compactness and a variety of morphologies.
As to a laptop computer, a battery size significantly influences a thickness of the computer. Accordingly, in order to reduce a thickness of the laptop computer, a number of studies and experiments for development of various shapes of batteries as well as attaining high capacity and performance thereof are currently being conducted. Especially, as interest in environmental problems is increased, a great deal of research on electric vehicles and hybrid electric vehicles has been conducted in order to replace conventional automobiles using fossil fuels such as gas-oil vehicles, diesel vehicles, etc., which are a major cause of air pollution.
Although an anode active material for a lithium secondary battery has been generally prepared using a carbon material, lithium metal or sulfur compounds have also been proposed. As to cathode active materials for a lithium secondary battery, lithium containing cobalt oxide (LiCoO2) is widely used. Additionally, other lithium transition metal oxides such as lithium containing manganese oxides such as LiMnO2 with a lamellar crystal structure, LiMn2O4 with a spinel crystal structure, etc., and lithium containing nickel oxide (LiNiO2) may also be used.
However, high energy density means possible exposure to risks and risks such as ignition, explosion, etc. may become more serious as energy density is increased. A lithium secondary battery as a major secondary battery has a drawback of inferior safety. For instance, when a battery is over-charged to about 4.2V, a cathode active material is degraded while dendrite growth of lithium metal and decomposition of an electrolyte may occur at an anode side. Furthermore, when excessive current flows in a short time due to over-charge, external short, nail penetration, local crushing, etc., the battery may be heated by IR heat generation, thus causing ignition/explosion thereof.
Increase in temperature of a battery promotes reaction between an electrolyte and an electrode. Then, heat of reaction is radiated, and the temperature of the battery is further increased, in turn accelerating the foregoing reaction. Owing to such a vicious circle, a phenomenon called thermal runaway, which is a very rapid temperature rise of the battery, may occur and battery ignition may be caused if the temperature increases to a certain level. As a result of the reaction between the electrolyte and the electrode, gas is also generated and internal pressure of the battery is increased, in turn causing battery explosion if the internal pressure reaches a certain level. Consequently, ignition/explosion risks as described above are a serious defect of conventional lithium secondary batteries.
Therefore, the most important consideration in development of improved lithium secondary batteries is to ensure safety. As a part of efforts to ensure safety of a battery, conventional techniques have been used and these are generally classified into a cell having an electric/electronic element fixed to an outer face thereof and use of materials contained in a cell. The electric/electronic element used in the former may include, for example, a PTC device using variation in temperature, a CID device, a protective circuit using variation in voltage, a safety vent using variation in internal pressure of a cell, etc. On the other hand, the latter may include addition of particular materials capable of being physically, chemically and/or electrochemically altered depending on variation in temperature and/or voltage of a cell.
Electric/electronic elements fixed to an outer face of a cell generally utilize temperature, voltage and/or internal pressure, thus being securely heat-shielded. However, such elements have disadvantages in that alternative assembly process and space are required and, for example, a CID device is employed only in a tubular cell. In addition, it is known that the foregoing elements are unsatisfactory to protect the cell under some conditions requiring fast response, that is, internal short, nail penetration, local crush, etc.
Alternatively, as to use of materials contained in a cell, a desirable additive may be added to an electrolyte or an electrode in order to improve safety thereof. A chemical safety device does not need additional assembly process or space and may be applicable to any type of batteries. However, due to addition of alternative materials, internal resistance of a cell is increased, in turn deteriorating cell performance.
Accordingly, there is still a strong requirement for development of a novel chemical safety method which can prevent ignition/explosion of a cell without deterioration in overall characteristics and/or performance thereof.